Multi pressure mode gas turbine

ABSTRACT

A gas turbine which is capable of operating in more than one pressure mode. The gas turbine, which may include a single fixed spool, multiple fixed spools, or a combination of fixed spool(s) and a free turbine, can operate in a positive pressure mode, a transatmospheric mode, or a subatmospheric mode. Valving is provided to control the particular pressure mode of operation in response to system requirements and to switch between pressure modes as required.

PRIORITY INFORMATION

This application claims the benefit of U.S. Provisional Application No.60/268,387 filed Feb. 13, 2001, the entire disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

This invention relates to the general field of gas turbines and moreparticularly to a gas turbine operating in more than one pressure mode.

BACKGROUND OF TIHE INVENTION

A gas turbine would include a turbine, a compressor, and a combustor,plus a recuperator (heat exchanger) where higher efficiencies aredesired in low pressure ratio gas turbines. Basically, air is compressedin the compressor, heated in the recuperator, mixed with fuel and burnedin the combustor, and then expanded in the turbine. The turbine drivesthe compressor and the turbine exhaust or discharge provides the heatfor the recuperator to heat the compressed air from the compressor.

With high power-to-weight ratios, high reliability, and low maintenance,gas turbines dominate commercial and military aircraft propulsion. Theyalso dominate aircraft auxiliary power units and large military tankpropulsion. But when it comes to commercial and personal land vehicles,such as buses, trucks and passenger cars, gas turbine applications havebeen extremely limited.

While virtually every automobile and small gas turbine manufacturer inthe United States, Europe and Japan has built and tested gas turbinepropulsion prototypes, none have seen production in commercially viablequantities. The problems have always been initial cost, fuelconsumption, and response time.

When a gas turbine is used with an electrical generator, the combinationis generally referred to as a turbogenerator; with the smaller versionscalled a microturbine. In a microturbine, the generator would normallybe a permanent magnet rotor rotatably driven by the turbine within anelectrical winding stator.

Microturbines are being successfully used in production commercial landvehicles. The Capstone Turbine Corporation of Chatsworth, Calif. nowproduces a microturbine, which is the primary source of power in hybridelectric buses and similar vehicles. These microturbines areeconomically sound while demonstrating long life with dramaticallyreduced maintenance and emissions. While production of these vehicles isstill limited today, it is increasing and the microturbine is beingproven in over-the-road revenue service.

However, to reach high production in a broad range of vehicles,including passenger cars, it is recognized that certain improvementswill be needed. These include lower initial cost, significantly fasterresponse time, and higher efficiency into the forty percent (40%) rangeat both full load and part load.

So why are microturbines successful in hybrid electric buses? First ofall, cost is far less of an issue because electric buses are expensiveand microturbines are a small percentage of the cost. In addition,electric buses have limited range, especially when air-conditioned. Theymust carry batteries that are typically one third of the weight of thebus. Microturbines provide essentially unlimited range, even when theair conditioning is on, and allow for a smaller, lighter, less expensivebattery pack.

Next, the traditional problem of fuel consumption is greatly amelioratedbecause the microturbine can always operate at its most efficient point.Even when the bus is stopped, the output can be used to charge thebatteries. In addition, the microturbine can be relatively small as itneed only provide the average power. Most of the peak power required toaccelerate the bus comes from the battery. The net result is that fuelconsumption is typically one half and sometimes one third of that ofconventional buses. Additionally, the problem of response time iseliminated because the battery provides the surge of power necessary toaccommodate sudden loads.

It must also be recognized that the single most important attribute ofthe microturbine is its low emission levels. The California AirResources Board (CARB) has approved fourteen manufacturers to sellheavy-duty diesel engines under 400 hp in California. NOx emissions ofthe best of these engines range between 3.2 and 3.8 g/bhp/hr CARB hascertified the diesel-fueled Capstone Turbine microturbine at less thanone quarter of these levels or 0.7 g/bhp/hr using no catalysts or aftertreatment, where “g” represents grams, “bhp” is brake horsepower and“hr” is hours.

Similarly, CARB has certified ten manufacturers to sell natural gas andLPG fueled engines. The NOx emissions of the best of these engines rangebetween 1.3 and 2.4 g/bhp/hr or roughly double that of the CapstoneTurbine microturbine operating on diesel. CARB has certified theCapstone microturbine at 0.53 g/bhp/hr on LPG and 0.26 g/bhp/hr onnatural gas.

Microturbines have also been successfully applied in a wide range ofapplications other than hybrid vehicles with over one thousand sevenhundred Capstone Turbine units delivered to date.

Capstone has shipped more than 1,700 microturbines to customersworldwide and these microturbines have logged more than 1,000,000 hoursof commercial operation. Individual units have run for more than 20,000hours in non-vehicular applications, such as distributed generation,with no maintenance other than fuel filter and air filter changes, withsome of these runs at maximum output. Considering if these units werepowering automobiles at only 80 kph (50 mph), this would be theequivalent of running 1,600,000 kilometers (1,000,000 miles) withessentially zero maintenance.

Existing microturbines drive permanent magnet generators that areintegrated into the basic design and therefore have no mechanical drivecapabilities. Fortunately, electric propulsion systems are now welldeveloped and very efficient over a broad range of speeds. In any event,much and sometimes most of the energy consumed by a bus is forauxiliaries such as air conditioning, air compressors, lights and fansthat can be driven electrically. And the thrust in all vehicles,including automobiles, is towards larger electric loads as all-electricpower steering, power brakes and other auxiliaries are developed.

Hybrid electric buses are an excellent application for existingmicroturbines and batteries. However, if production of microturbines isto reach automotive quantities, the bulk, weight and cost of the batterypack must go. When the microturbine has to operate without an energystorage system (battery), higher efficiency at full load will be needed.But, even more important, with no battery, much higher efficiency atpart load will be critical. In addition, the response time must beimproved and the initial cost must drop.

For purposes of discussion, all calculations will be based on thefollowing assumptions unless otherwise noted:

Ambient temperature: 15° C. (59° F.) Air flow 0.600 kg/sec (1.323lb/sec) Compressor pressure ratio 4.00:1 Turbine pressure ratio: 3.76:1Turbine Inlet temperature: 875° C. (1607° F.) Compressor efficiency:0.780 Turbine efficiency: 0.850 Recuperator effectiveness: 0.850Combustor efficiency: 0.995 Mechanical efficiency: 0.990 Generator &inverter efficiency: 0.900 Radiation losses: 0.005 Compressor bleed:0.000 Fuel: #2 diesel

No allowance is made for change in pressure drop due to differences inrecuperator effectiveness, as this is a function of recuperator design.

A microturbine using the above parameters would produce 74.3 kW atthirty-one and one half percent (31.5%) efficiency. While themicroturbine is a very simple device conceptually, in practice, it hasvery sophisticated engineering. Thus, except for the fuel pump andpossibly a cooling fan for the electronics, the only moving part is therotor group, which includes the turbine wheel, compressor wheel andpermanent magnet rotor. When the rotor group or spool is mounted oncompliant foil fluid film bearings, there is no lubrication system and,indeed, no oil, no oil pump, no oil cooler and no need for oilservicing. As the microturbine is air cooled, there are no fluids in themachine other than fuel, and no turbine-driven accessories. This createsa compact package that operates over a limited speed range and is wellsuited to vehicles using electric propulsion.

In large gas turbines, the roads to high power and efficiency are: 1)increasing component efficiencies, 2) increasing pressure ratios, and 3)increasing turbine inlet temperatures. Unfortunately for small gasturbines such as microturbines, efficiencies of small components willnever be as high as those of large components. Also compressors withsmall airflows cannot be designed with as high a pressure ratio ascompressors with large airflows, and still be efficient. Finally, theturbine inlet temperature is limited by the use of the recuperator thatsmall gas turbines must use if they wish to have competitiveefficiencies.

Using a recuperator, a heat exchanger that transfers heat from the gasturbine's exhaust to the compressor discharge air before this air goesinto the combustor, microturbine efficiency can be improved by reducingthe fuel required. The ratio of recuperator air inlet temperature minuscompressor discharge temperature to turbine discharge temperature minuscompressor discharge temperature is known as recuperator effectiveness.Eighty-five percent (85%) is a typical goal and in a typical small gasturbine, this will halve the fuel consumption or double the efficiency.

The disadvantages to using recuperators are: 1) they are heavy, oftendoubling the weight of the microturbine, 2) they are expensive, far andaway the most expensive component in the microturbine and 3) they arelimited in the temperature that they can take. As the recuperator inlettemperature is the same as the turbine discharge temperature, theturbine discharge temperature must also be limited. As the turbine inlettemperature is related to the turbine discharge temperature by thepressure ratio and turbine efficiency, the turbine inlet temperaturemust also be limited. This effect is even more pronounced at part loadswhere the rpm is reduced thus lowering the pressure ratio. Limiting theturbine inlet temperature limits the power and efficiency of themicroturbine.

SUMMARY OF THE INVENTION

The present invention is directed to a gas turbine that operates in morethan one pressure mode. During various system operating requirements,the gas turbine may operate in a positive pressure mode, atransatmospheric pressure mode, or a subatmospheric pressure mode.Valving is provided to control the particular pressure mode of operationin response to system requirements and to switch between pressure modesas required. The gas turbine may include a single fixed spool, multiplefixed spools, or a combination of fixed spool(s) and a free turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the present invention in general terms, referencewill now be made to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of a positive pressure, recuperated,gas turbine;

FIG. 2 is a schematic block diagram of a subatmospheric pressure,recuperated, gas turbine;

FIG. 3 is a schematic block diagram of a multi pressure mode, singlespool, gas turbine of the present invention;

FIG. 4 is a schematic block diagram of a multi pressure mode, singlespool, intercooled, gas turbine of the present invention;

FIG. 5 is a schematic block diagram of a positive pressure, two spool,recuperated intercooled, gas turbine;

FIG. 6 is a schematic block diagram of a multi pressure mode, two spool,recuperated, intercooled, gas turbine of the present invention;

FIG. 7 is a schematic block diagram of a multi pressure mode, spool andone half, recuperated, gas turbine of the present invention with a freeturbine;

FIG. 8 is a schematic block diagram of a multi pressure mode, spool andone half, recuperated, intercooled, gas turbine of the present inventionwith a free turbine;

FIG. 9 is a schematic block diagram of a multi pressure mode, two andone half spool, recuperated, intercooled gas turbine of the presentinvention;

FIG. 10 is a schematic block diagram of an alternate multi pressuremode, two and one half spool, recuperated, intercooled gas turbine ofthe present invention;

FIG. 11 is a schematic block diagram of the multi pressure mode gasturbine of FIG. 10 and having three way valves in a high powerconfiguration;

FIG. 12 is a schematic block diagram of the multi pressure mode gasturbine of FIG. 10 and having three way valves in a mid powerconfiguration;

FIG. 13 is a schematic block diagram of the multi pressure mode gasturbine of FIG. 10 and having three way valves in a low powerconfiguration;

FIG. 14 is a schematic block diagram of the multi pressure mode gasturbine of FIG. 10 and having four way valves in a high powerconfiguration;

FIG. 15 is a schematic block diagram of the multi pressure mode gasturbine of FIG. 10 and having four way valves in a mid powerconfiguration; and

FIG. 16 is a schematic block diagram of the multi pressure mode gasturbine of FIG. 10 and having four way valves in a low powerconfiguration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to understand the present invention, it is necessary tocomprehend the various possible pressure operating modes of a gasturbine. Gas turbines that burn fuel must take in air for combustion anddischarge the products of combustion. Thus, if fuel is burned inside thegas turbine, the cycle cannot be a closed loop and must be openedsomewhere. The cycle can be opened in various places.

In a conventional, positive-pressure, recuperated gas turbine 10, asillustrated in FIG. 1, the compressor 11, turbine 12 and generator 13are all on a common shaft 14. The compressor 11 compresses air for thecombustor 16 after it has been heated in the recuperator 18. The heatedcompressed air is mixed with fuel in the combustor 16 where it is thanignited and burned. The combustion products are then expanded in theturbine 12 which drives the compressor 11 and generator 13. This cycleis opened between the discharge 15 from the low-pressure side of therecuperator 18 and the compressor inlet 17. Thus, ambient air, that mayhave been filtered, cooled or otherwise treated, enters the compressor11 and the products of combustion are discharged out of the low-pressureside of the recuperator 18.

In the subatmospheric cycle gas turbine 19 illustrated in FIG. 2,ambient air enters the recuperator 18 where it is preheated beforeentering the combustor 16. The hot products of combustion leave thecombustor 16 and discharge through the turbine 12 into a partial vacuumthat is sustained by the compressor 11. Before entering the compressor11, the hot gases are cooled by the recuperator 18 with the rejectedheat being used to preheat the ambient air entering the recuperator 18.In some cases, the air leaving the low-pressure side of the recuperator18 is further cooled with an intercooler (not shown) before entering thecompressor 11. Cooling may be necessary since the amount of power thatthe compressor 11 takes and the amount of air that enters the compressor11 are functions of the compressor inlet temperature. This cycle is openbetween the compressor outlet 30 and the inlet 37 to the high-pressureside of the recuperator 18.

A very important side effect of operating the gas turbine 19subatmospherically is that the mass flow through the gas turbine 19 isreduced by a factor roughly equal to the pressure ratio, and maximumpower is reduced by approximately the same ratio. Thus, as an example,although the basic system components are essentially the same in bothcycles, if the pressure ratio is four to one, the maximum power of thegas turbine 19 operating subatmospherically (FIG. 2) will be roughly onequarter that of the same gas turbine 10 operating in a conventionalpositive pressure mode (FIG. 1).

Gas turbines (turbogenerators) that drive constant-speed loads such asconventional fixed-frequency generators cannot slow down at reducedloads because the compressor 11 and turbine 12 are geared to thegenerator 13 that must maintain constant speed to maintain constantfrequency. At low loads, the fuel flow is reduced but the airflow isnot. This results in low turbine inlet temperatures that result in lowefficiencies.

If the load does not require constant speed, as when the output of thegenerator 13 is conditioned by an inverter (not shown), the gas turbinecan be slowed down when the load is low resulting in much betterpart-load efficiency. However, if a load is suddenly applied, the gasturbine will be unable to meet the increased load until it hasaccelerated to the appropriate rpm. Thus the gas turbine must choosebetween operating at high rpm and having poor efficiency and operatingat low rpm and being unable to meet a step load.

If the gas turbine could be transitioned rapidly between asubatmospheric pressure mode (FIG. 2) and a positive pressure mode (FIG.1), this problem could be greatly ameliorated. The gas turbine couldoperate at high rpm in the subatmospheric pressure mode when the load islow and when the load is rapidly increased, the gas turbine couldrapidly transition to the positive-pressure mode and meet the load sincethe gas turbine would already be at high rpm.

For purposes of illustration, a generator 13 has been shown as the loadfor the gas turbine. It should be recognized, however, that the loadcould be anything that is driven mechanically, hydraulically orpneumatically.

In its simplest form, the gas turbine 25 of the present invention isillustrated in FIG. 3. It is a shown as a single spool or rotor groupmachine, with the single spool including compressor 11, turbine 12 andgenerator 13. The gas turbine 25 includes three high power valves 20 andthree low power valves 21 which are normally closed.

In the high power or positive-pressure mode, the high power valves 20are open while in the low power or subatmospheric pressure mode, the lowpower valves 21 are open.

Referring to FIG. 3, with the high power valves 20 open, ambient airenters the compressor 11, goes through the recuperator 18, combustor 16,turbine 12 and low-pressure side of the recuperator 18 before exiting.However, if the load is low, the high power valves 20 close and the lowpower valves 21 open. The ambient air then enters the recuperator 18,goes through the cycle and is discharged to atmosphere out of thecompressor 11. Power is reduced, mass flow is reduced, the gas turbine25 operates at high rpm, and good efficiency is maintained. If a stepload is suddenly applied, the gas turbine reverts to positive-pressuremode (high power valves open, low power valves closed) essentiallyinstantaneously and handles the load.

It is important to note that the temperatures in the hot section of thegas turbine 25 including the recuperator 18, the combustor 16 and theturbine 12 are essentially unchanged during the transition. There is nothermal shock even though the loads have changed. It should also benoted that there is no change in rpm during the transition, thusminimizing any mechanical shock.

FIG. 4 illustrates a single spool gas turbine 26 which includes anintercooler 31 to provide additional cooling of the compressor inlet airbetween low pressure side of the recuperator 18 and the compressor 11.

A positive pressure, two spool, recuperated, intercooled, gas turbine isillustrated in FIG. 5. The first spool or rotor group includescompressor 11, turbine 12, and generator 13 on shaft 14. The secondspool or rotor group includes compressor 33, turbine 34, and generator41 on shaft 35.

Compressor 11 provides compressed air to compressor 33 throughintercooler 38. The compressed air from compressor 33 is heated inrecuperator 18 before being mixed with fuel and burned in combustor 16which provides combustion products to turbine 34. The exhaust gases fromturbine 34 are burned in combustor 43 before being expanded in turbine12 whose exhaust proceed to recuperator 18.

The high-pressure rotor produces almost as much power as thelow-pressure rotor. The mass flows are the same, give or take any bleedor the addition of any fuel. The pressure ratios are optimized to be thesame. The turbine inlet temperatures are the same. The only significantdifference is that the low-pressure compressor 11 sees ambienttemperature air while the high-pressure rotor compressor 33 sees air ata slightly elevated temperature because the intercooler is not 100%effective. If the low-pressure compressor 11 sees an air inlettemperature of 15° C. (59° F.), then the high-pressure compressor 33might see an air inlet temperature of 25° C. (77° F.). Then, if thelow-pressure rotor group produces 74.3 kW, the high-pressure rotor groupshould produce 70.9 kW for a total of 145.2 kW.

The key point is that doubling the number of spools or rotors hasessentially doubled the power but there is still only one recuperator18, the most expensive component in the system. Thus the specific costis lower. While an intercooler 38 has been added, it is essentially thesame as a turbocharger aftercooler and is far less expensive to buildthan a high temperature recuperator.

The original objective of the recuperator 18 was to raise thetemperature of the compressor discharge air to as close to the turbinedischarge temperature as possible, thus reducing the amount of fuelrequired by the combustor 43. The air entering the combustor 43 for thelow-pressure rotor is, however, already at the turbine dischargetemperature. So the low-pressure rotor operates with the thermodynamicequivalent of a 100% effective recuperator 18. Thus the low-pressure,74.3 kW rotor now operates with a theoretical efficiency of thirty-eightand three tenths percent (38.3%) whereas the 70.9 kW, high-pressurerotor operates with the original theoretical efficiency of thirty-oneand one half percent (31.5%). Thus the total theoretical efficiency,weighted for power output, is thirty-five percent (35.0%). The netresult is that the efficiency of the recuperated intercooled gas turbinewill be substantially more than that of a conventional recuperated gasturbine.

FIG. 6 illustrates a two-spool or two rotor group gas turbine 32 of thepresent invention. It should be noted that the gas turbine 32 does notinclude combustor 43 between turbine 12 and turbine 34. Likewise, thesecond spool or rotor group of shaft 35 does not include a generator.The gas turbine 32 operates with the high power valves 20, high/midpower valve 44, and high/low power valve 45 all open for high power withboth rotor groups operating in the positive-pressure mode. As the loadis reduced, the mid power valves 22 and mid/low power valve 46 areopened and the high power valves 20 and high/low power valve 45 areclosed with high/mid power valve 44 remaining open. In thisconfiguration, gas turbine 32 operates in the mid power ortransatmospheric pressure mode.

Air enters between the two compressors 11, 33 and the upper rotor groupof shaft 35 operates positive-pressure while the lower rotating group ofshaft 14 operates sub atmospherically. Both rotor groups have their massflow reduced by a factor roughly equal to the pressure ratio of thecompressor 11 with a corresponding reduction in power.

If the load drops further, the mid power valves 22 and the high/midpower valve 44 are closed and the low power valves 21 and high/low powervalve 45 are opened with mid/low power valve 46 remaining open. In thiscase both rotor groups now operate subatmospherically with a furtherreduction in power.

While it is accurate to state that the gas turbine operates in asubatmospheric pressure mode of less than one atmosphere, it isrecognized that a miniscule amount of positive pressure is necessary toallow the compressor to discharge into the atmosphere. Also, while it isaccurate to state that the gas turbine operates in a transatmosphericpressure mode, it is recognized the high pressure turbine discharge andthe low pressure turbine inlet may be at either a positive pressure or asubatmospheric pressure. The high pressure turbine inlet will be at apositive pressure and the low pressure turbine discharge will be at asubatmospheric pressure.

Thus, there are three modes with three separate power levels at whichhigh efficiency can be obtained. If, as an example, the pressure ratioof each of the compressors was four to one, close to full efficiencycould be achieved at 100% power, 25% power and 6¼% power. Yet thetransition between operating at these powers would be essentiallyinstantaneous. This configuration not only increases efficiency but alsoreduces the equipment cost per horsepower. The fact that the equipmentcost per horsepower is reduced is derived from the use of a second spoolto increase power without adding recuperator surface as well as theability to increase turbine inlet temperature withoutrecuperator-imposed limitations.

In essence, the original rotor group (the low-pressure rotor) isturbocharged or supercharged by the second spool or rotor group (thehigh-pressure rotor) in series with the original rotor group. Anintercooler 38 is placed between the two compressors 11, 33 because thepower required to compress air is a function of temperature. Thehigh-pressure rotor is smaller than the low-pressure rotor as it sees amuch smaller volume of air because the air is compressed.

As previously stated, the turbine inlet temperature is limited becausethe turbine discharge temperature must not be too high for therecuperator. This limitation is now eliminated for the high pressurerotor since the turbine discharges into a combustor inlet rather thaninto the recuperator. Therefore, it is now feasible to increase theturbine inlet temperature of the high pressure rotor by increasing thefuel flow into the high pressure combustor. Increasing the turbine inlettemperature of the high pressure rotor by 100° C. (180° F.) from 875° C.(1607° F.) to 975° C. (1787° F.) increases the power of the highpressure rotor from 70.9 kW to 86.5kW.

While this 15.6 kW increase in power is accompanied by an increase infuel flow of 64.2 kg cal/hr (255 kBtu/hr), the turbine dischargetemperature is increased from 602° C. (1116° F.) to 683° C. (1261° F.).As the turbine discharges directly into the low pressure combustor, thelow pressure combustor sees a higher combustor inlet temperature andthus needs less fuel to meet the required low pressure turbine inlettemperature. In simple terms, the energy in the additional fuel isconverted into additional turbine power and into additional turbineexhaust heat. Since the additional turbine exhaust heat reduces theamount of fuel required by the low pressure combustor on a one-for-onebasis, the net incremental fuel of both combustors is only increased bythe amount of energy that is converted into additional turbine power. Inother words, the additional fuel consumed is converted at essentially100% efficiency into useful power, give or take the small amount of heatthat is radiated or conducted away.

Unfortunately, there is a limit to the temperature that the turbine cantake and this limit is being rapidly approached for uncooled metalturbine wheels. Unfortunately, the turbine wheel in a typicalmicroturbine is too small for blade cooling. The answer is ceramics. Thehigh-pressure rotor is small and is conceptually similar to that of aturbocharger. Small ceramic turbochargers have been built for vehicularapplications in quantities of hundreds of thousands.

The rationale for ceramic turbochargers was not primarily to allow forhigher inlet temperatures. The first goal was to reduce the rotatingweight, thus eliminating “turbo lag” and permitting the turbocharger toaccelerate rapidly. The second goal was to reduce the mass and thereforethe energy in the event of a wheel failure. One of the most expensiveparts in a turbocharger is the turbine housing, which must offercontainment. Reducing the energy in the wheel allows for a lighter andless expensive housing. Both of these attributes would also be helpfulin a microturbine. However, the most important thing would be theability to handle high temperatures.

Using a ceramic turbine wheel and raising the turbine inlet temperatureto 1214° C. (2217° F.) would increase the high-pressure turbinedischarge temperature to 875° C. (1607° F.) thus eliminating anylow-pressure combustor. More important, the power produced by thehigh-pressure rotor would be 125.5 kW. Added to the 74.3 kW output ofthe low-pressure rotor, the total output would be 199.8 kW, which roundsto 200 kW. Fuel consumption in what is now the only combustor would be428.3 kg cal/hr (1700 kBtu/hr) for an efficiency of forty and one tenthpercent (40.1%).

A one and one half spool gas turbine 28 is shown in FIG. 7. The gasturbine 28 includes a half spool or rotor group which includes freeturbine 23 and generator 24 on a common shaft 27. The one half spool isseparate from the spool or rotor group which includes compressor 11 andturbine 12. FIG. 8 is a one and one half spool gas turbine 29 whichincludes intercooler 31.

The free turbine 23 enables the gas turbines 28, 29 to handle step loadswithout external stored energy. Separate turbines 12, 23 are thus usedto power the compressor 11 and the generator 24, respectively. Theseseparate turbines 12, 23 are independent and operate at different andvarying speeds. The first turbine 12 and the compressor 11 that itpowers form the core of the gas producer section of either of gasturbines 28, 29. The turbine providing the output power is the freeturbine 23.

If a step load is suddenly applied, the gas producer section is free toaccelerate rapidly, unconstrained by the load on the generator 24.Typically, the gas producer in a small gas turbine might accelerate fromidle to maximum power in perhaps five seconds or roughly 20% of fullpower per second. In the meantime, the generator 24 does provide theneeded electricity for a brief period by using its own inertia alongwith that of the power turbine 23 to provide the power while it isslowing down. Fortunately the torque curve of a power turbine 23 is suchthat torque increases dramatically as it slows down. Typically stalltorque will be 2½ or more times the full-load torque. The mass and speedof the power turbine 23 and generator 24 will determine how many secondsat what power are available. In effect, the power turbine 23 andgenerator 24 act as a flywheel to handle step loads. A key point is thateven if the generator 24 is overloaded, the gas turbine cannot bestalled or damaged.

One major positive aspect to free turbines is the ease with which theycan be used for mechanical propulsion. The ability to run with amechanical drive, as virtually all vehicular gas turbine have done inthe past, is important. Free turbines have the equivalent of a built intorque converter. As noted above, the stall torque is typically morethan 2½ times the torque at full power. A typical free turbine candeviate plus or minus 25% from maximum power rpm and only lose 5% of itsoutput power.

Two and one half spool gas turbines 48 and 36 are illustrated in FIGS. 9and 10, respectively. The two and one half spool gas turbinesessentially combines the one and one half spool gas turbines 28, 29 ofFIGS. 7 and 8, with the two spool gas turbine 32 of FIG. 6.

In FIG. 9, the free turbine 23 is between the high pressure turbine 34and the low pressure turbine 12 in receiving the combustion productsfrom the combustor 16. In FIG. 10 the high pressure turbine 34 receivesthe combustion products from combustor 16, exhausts to the low pressureturbine 12 which is turn exhausts to the free turbine 23 before thecombustion products are provided to recuperator 18.

While FIGS. 3-4 and 6-10 illustrate the use of discrete valves, itshould be recognized that the number of valves can be reduced bycombining valves into three-way or four-way valves. Gas turbine 40 ofFIGS. 11-13 illustrates the use of three-way valves in a high power, midpower, and low power configuration, respectively. FIGS. 14-16illustrates gas turbine 42 having four-way valves in a high power, midpower, and low power configuration, respectively.

For the purpose of this patent application, we define a three-way valveas one in which the flow passage is open between two of three openingswith the third opening sealed shut, and that the valve can alternatebetween these two openings to determine which one is open and which isshut. The use of three-way valves can reduce the number of valves fromnine open/close valves to four three-way valves 50, 51, 52, and 53 plustwo open/close valves, namely low pressure valve 21 and mid pressurevalve 22, as shown in FIG. 11-13.

The valves are shown as pivoting arrowheads or tails, which will move upor down (or right or left) as required. These valves would always be inone of the extreme positions unless the valves were in transition. Inthe high, medium and low power settings, they would connect to H, M orL, respectively. The open/shut valves labeled M and L would be normallyclosed and opened only for medium or low power settings, respectively.

In the high power configuration of FIG. 11, air enters compressor 11through three-way valve 52 and exhaust gas leaves the gas turbine 40through three-way valve 50. In the mid power configuration of FIG. 12,air enters through open/close valve 22 while the compressor 11discharges air through the same three-way valve 53. In the low powerconfiguration of FIG. 13, air enters recuperator 18 through three-wayvalve 51 and is discharged from compressor 33 through open/close valve21.

For the purpose of this patent application, we define a four-way valveas one in which the flow passage is open between two of three openingsas in the three-way valve, but the third opening is connected toatmosphere. As with the three-way valve, the four-way valve canalternate between these two openings to determine which two areconnected while the remaining opening is connected to the atmosphere.The use of four-way valves can reduce the number of valves from nineopen/close valves to four four-way valves 60, 61, 62, and 63 as shown inFIGS. 14-16.

The control of a microturbine is generally described in U.S. Pat. No.6,023,135 issued Feb. 8, 2000 to Mark G. Gilbreth et al. and entitled“Turbogenerator/Motor Control System” and U.S. Pat. No. 6,031,294 issuedFeb. 29, 2000 to Everett R. Geis et al. and entitled“Turbogenerator/Motor Controller with Ancillary Energy Storage”, both ofwhich are incorporated herein by reference.

The control of the gas turbine 42 of FIGS. 14-16 is relatively simple.Based upon information from the generator 24 (or elsewhere in the gasturbine), the control 56 provides a fuel signal 65 to the fuel valve 55to adjust fuel flow as required, plus operational commands or signals66, 67, 68, 69 to four-way valves 60, 61, 62, 63 respectively.

In the multi pressure mode gas turbine 42 of FIGS. 14-16, if more poweris needed, the appropriate four-way valves are triggered to increase gasturbine pressure. If the power demand is such that the gas turbine canprovide the power demand at a lower gas turbine pressure, theappropriate valves are triggered to decrease gas turbine pressure. Otherthan the shifting of valve positions to change gas turbine pressure, thecontrol of a multi pressure mode gas turbine 42 is substantiallyidentical to the control of any single pressure or conventional gasturbine regardless of whether it is free turbine or fixed shaft.

In the high power position, the operation is the same as with thethree-way valves. In the medium power position, the operation is alsothe same, except that the ambient air now enters through the openinglabeled HL that is now connected to the atmosphere. Correspondingly, inthe low power setting, the air exits through the passage labeled HM thatis now connected to the atmosphere.

Further, it should be noted that additional rotor groups can be usedwith the same arrangement of valves to further increase the number ofpower levels at which high efficiency can be achieved. Also, there areno simply discrete points at which high efficiency can be achieved. Ineach mode, the gas turbine can operate at any power setting up to themaximum for that mode at a higher efficiency than it would have in anyof the higher pressure modes.

While the free turbine goes a long ways towards solving the step loadproblem, it simply cannot handle an instantaneous step from very lowloads to full load. This is where the transatmospheric pressure andsubatmospheric pressure operating cycle comes in.

Whether a gas turbine runs in the conventional positive-pressure mode,the transatmospheric pressure mode, or the subatmospheric pressure mode,the hardware is essentially the same. The differences lie in whereambient air enters the cycle and where the combustion products aredischarged. If ambient air enters the cycle either between thecompressors or at the recuperator inlet, the cycle pressure and massflow will be lower as opposed to the air entering the cycle in theconventional manner upstream of the compressor. Thus the power developedwill be correspondingly less.

One disadvantage to the transatmospheric pressure cycle and thesubatmospheric pressure cycle is that one or both of the compressorsmust compress the products of combustion. A second disadvantage is thatthese products do not enter one or both of the compressors at ambienttemperature. However, in the subatmospheric pressure cycle, the airentering the recuperator on the cold side is at ambient temperatureinstead of at compressor discharge temperature as it would be in aconventional cycle. Accordingly, the recuperator can be quite effectivein cooling the combustion products before they enter the compressor. Inaddition, an intercooler can be added.

If the interstage pressure between the two spool compressors could bereduced to atmospheric then the high-pressure spool or rotor would beoperating at positive but dramatically reduced pressure and thelow-pressure spool or rotor would be operating subatmospherically.

Assuming that the individual compressor pressure ratios were 4:1, themass flow would be reduced by a factor of four and therefore the maximumcycle power would be achieved with high efficiency at 25% of normal fullpower. Equally important, the rotor groups would be operating at fullrpm at 25% power.

If the gas turbine could be operated in the full subatmospheric pressuremode where the highest pressure in the cycle is atmospheric, the powerwould drop by a further factor of four. Maximum cycle power wouldtheoretically be achieved at 6¼% of normal full power. Relatively highefficiency would also be achieved at this power setting. Thus, if thegas turbine were rated at 200 kW (267 hp), it should have fullefficiency at 200 kW (267 hp), and reasonably close to full efficiencyat 50 kW (67 hp) and 12½ kW (17 hp). It is easy to image a bus creepingthrough city traffic with loads varying as air conditioning and otheraccessories cycle on and off, and then suddenly requiring power foracceleration. Keeping the rpm up is the key to handling these steploads. If the microturbine can transition between the various modes,then it can operate at low loads efficiently and still be able to handlestep loads.

As a practical matter, the parasitic losses will remain about the samefor all three modes and constitute a higher percentage of the cyclepower in the reduced-pressure modes. However, parasitic losses arerelatively low in machines that use compliant foil fluid film bearings,have no engine-driven accessories, and require no gears. Anotherconsideration is that, as noted above, one or both of the compressorswill see products of combustion and higher inlet temperatures in thereduced-pressure modes.

Fast acting high pressure valves of the type required for the operationof this invention are commercially available and have been usedextensively in systems such as aircraft pneumatic actuation systems,including engine thrust reverser actuation systems, nozzle controls,flap actuation systems, weapon ejection systems and gun drive systems.In addition, pneumatic actuated turbocharger bypass valves would also besuitable.

The transition time between operating in the various pressure modes willbe as fast as the valves can operate. Note that in transitioning betweenmodes, there is no significant change in temperature anywhere in thecycle even though the change in power is dramatic.

Microturbines must be able to handle situations where the load isinstantly lost. This can be as simple as the main breaker openingunexpectedly or the driver going into regenerative braking mode. Eitherway, the stored heat in the recuperator wants to accelerate the unloadedmicroturbine, generally requiring either braking resistors or bleedvalves. With the ability to switch instantaneously to subatmosphericpressure operation, the power needing to be dissipated is reduced by afactor of at least sixteen.

The gas turbines of the present invention can have an efficiency over40% from fuel-in to useful electricity-out. Close to this efficiency canbe achieved over a wide range including very low power outputs. Severestep loads can be handled with ease. Yet the cost per kW of such amachine should be much less than that of existing microturbines. Theresult is substantially increased part-load efficiency and/or asubstantially reduced response time to a step load.

While specific embodiments of the invention have been illustrated anddescribed, it is to be understood that these are provided by way ofexample only and that the invention is not to be construed as beinglimited thereto but only by the proper scope of the following claims.

What I claim is:
 1. A method of operating a recuperated gas turbine,comprising: providing valving between the recuperator and the gasturbine compressor; setting the valving in a first configuration tooperate the gas turbine in a positive pressure mode; and setting thevalving in a second configuration to operate the gas turbine in asubatmospheric pressure mode.
 2. The method of operating a recuperatedgas turbine of claim 1, and in addition, setting the valving in a thirdconfiguration to operate the gas turbine in a transatmospheric pressuremode.
 3. The method of operating a recuperated gas turbine of claim 2,wherein the valving provided is a plurality of open/close valves.
 4. Themethod of operating a recuperated gas turbine of claim 2, wherein thevalving provided is a plurality of three-way valves and a plurality ofopen/close valves.
 5. The method of operating a recuperated gas turbineof claim 2, wherein the valving provided is a plurality of four-wayvalves.
 6. A method of operating a recuperated gas turbine, comprising:providing valving between the recuperator and the gas turbinecompressor; setting the valving to operate the gas turbine in a positivepressure mode at high power; and setting the valving to operate the gasturbine in a subatmospheric pressure mode at low power.
 7. The method ofoperating a recuperated gas turbine of claim 6, and in addition, settingthe valving to operate the gas turbine in a transatmospheric pressuremode at mid power between high power and low power.
 8. The method ofoperating a recuperated gas turbine of claim 6, and in addition,intercooling the recuperated air delivered to the gas turbinecompressor.
 9. The method of operating a recuperated gas turbine ofclaim 6, wherein the positive pressure mode is generally set from fullrated power to full rated power divided by the gas turbine compressorpressure ratio, and the subatmospheric pressure mode is generally set atfull rated power divided by the gas turbine compressor pressure ratio.10. The method of operating a recuperated gas turbine of claim 7,wherein the positive pressure mode is generally set from full ratedpower to full rated power divided by the pressure ratio of the highpressure gas turbine compressor, the transatmospheric pressure mode isgenerally set between full rated power divided by the pressure ratio ofthe high pressure gas turbine compressor to full rated power divided bythe product of the pressure ratio of the high pressure compressor timesthe pressure ratio of the low pressure compressor, and thesubatmospheric pressure mode is generally set at full rated powerdivided by the product of the pressure ratio of the high pressurecompressor times the pressure ratio of the low pressure compressor. 11.A method of operating a recuperated gas turbine including a freeturbine, comprising: providing valving between the recuperator and thegas turbine compressor; at high power, setting the valving to operatethe gas turbine in a positive pressure mode; and at low power, settingthe valving to operate the gas turbine in a subatmospheric pressuremode.
 12. The method of operating a recuperated gas turbine including afree turbine of claim 11, and in addition, at mid power, setting thevalving to operate the gas turbine in a transatmospheric pressure mode.13. A method of operating a recuperated two spool gas turbine,comprising: providing valving between the recuperator, the first spoolgas turbine compressor, and the second spool gas turbine compressor;setting the valving to operate the gas turbine in a positive pressuremode at high power; and setting the valving to operate the gas turbinein a subatmospheric pressure mode at low power.
 14. The method ofoperating a recuperated two spool gas turbine of claim 13, and inaddition, setting the valving to operate the gas turbine in atransatmospheric pressure mode at mid power between high power and lowpower.
 15. The method of operating a recuperated two spool gas turbineof claim 13, and in addition, intercooling the compressed air betweenthe first spool gas turbine compressor and the second spool gas turbinecompressor.
 16. The method of operating a recuperated two spool gasturbine of claim 13, and in addition, intercooling the recuperated airbetween the first spool gas turbine compressor and the recuperator. 17.The method of operating a recuperated two spool gas turbine of claim 13,and in addition, intercooling the compressed air between the first spoolgas turbine compressor and the second spool gas turbine compressor; andintercooling the recuperated air between the recuperator and the firstspool gas turbine compressor.
 18. A method of operating a recuperatedtwo spool gas turbine including a free turbine, comprising: providingvalving between the recuperator, the first spool gas turbine compressor,and the second spool gas turbine compressor; setting the valving tooperate both spools of the two spool gas turbine in a positive pressuremode at high power; setting the valving to operate one spool of the twospool gas turbine in a positive pressure mode and the other spool of thetwo spool gas turbine in a subatmospheric pressure mode at mid power;and setting the valving to operate both spools of the two spool gasturbine in a subatmospheric pressure mode at low power.
 19. The methodof operating a recuperated two spool gas turbine including a freeturbine of claim 18, and in addition, intercooling the compressed airbetween the first spool gas turbine compressor and the second spool gasturbine compressor.
 20. The method of operating a recuperated two spoolgas turbine including a free turbine of claim 18, and in addition,intercooling the recuperated air between the first spool gas turbinecompressor and the recuperator.
 21. The method of operating arecuperated two spool gas turbine including a free turbine of claim 18,and in addition, intercooling the compressed air between the first spoolgas turbine compressor and the second spool gas turbine compressor; andintercooling the recuperated air between the recuperator and the firstspool gas turbine compressor.
 22. A method of operating a recuperatedtwo and one half spool gas turbine, comprising: providing valvingbetween the recuperator, the first spool gas turbine compressor, and thesecond spool gas turbine compressor; at high power, setting the valvingto operate the gas turbine in a positive pressure mode; and at lowpower, setting the valving to operate the gas turbine in asubatmospheric pressure mode.
 23. The method of operating a recuperatedtwo and one half spool gas turbine of claim 22, and in addition, at midpower, setting the valving to operate the gas turbine in atransatmospheric pressure mode.
 24. A turbogenerator comprising: a gasturbine including a compressor, a turbine, a combustor, a recuperatorand a generator; said recuperator receiving compressed air from saidcompressor to be heated in said recuperator by the expanded exhaustgases from said turbine, said heated compressed air from saidrecuperator supplied to said combustor to be mixed with fuel andcombusted to provide combustion gases for expansion in said turbinewhich drives said compressor and said generator; valving disposedbetween said recuperator and said compressor to operate said gas turbinein a positive pressure mode at high power and to operate said gasturbine in a subatmospheric pressure mode at low power.
 25. Theturbogenerator of claim 24, and in addition, an intercooler disposedbetween said recuperator and said compressor.
 26. The turbogenerator ofclaim 24 wherein said valving is a plurality of open/close valves. 27.The turbogenerator of claim 24 wherein said plurality of open/closevalves is six, with three of said plurality of open/close valves open athigh power and the other three of said plurality of open/close valvesopen at low power.
 28. The turbogenerator of claim 27 wherein said threeof said plurality of open/close valves open at high power are at therecuperator exhaust, the compressor inlet, and between said compressorand said recuperator, and the other three of said plurality ofopen/close valves open at low power are at the recuperator inlet, thecompressor outlet and between the recuperator exhaust and the compressorinlet.
 29. The turbogenerator of claim 24 wherein said valving is aplurality of three-way valves and a plurality of open/close valves. 30.The turbogenerator of claim 24 wherein said valving is a plurality offour-way valves.
 31. A turbogenerator comprising: a gas turbineincluding a compressor, a turbine, a combustor, a recuperator and agenerator; said recuperator receiving compressed air from saidcompressor to be heated in said recuperator by the expanded exhaustgases from said turbine, said heated compressed air from saidrecuperator supplied to said combustor to be mixed with fuel andcombusted to provide combustion gases for expansion in said turbinewhich drives said compressor and said generator; valving disposedbetween said recuperator and said compressor to operate said gas turbinein a positive pressure mode at high power, to operate said gas turbinein a transatmospheric pressure mode at mid power, and to operate saidgas turbine in a subatmospheric pressure mode at low power.
 32. Theturbogenerator of claim 31, and in addition, an intercooler disposedbetween said recuperator and said compressor.
 33. The turbogenerator ofclaim 31 wherein said valving is a plurality of open/close valves. 34.The turbogenerator of claim 33 wherein said plurality of open/closevalves is nine, with two of said plurality of open/close valves open athigh power, one of said plurality of open/close valves open at both highand mid power, one of said plurality of open/close valves open at bothhigh and low power, two of said plurality of open/close valves open atmid power, one of said plurality of open/close valves open at both midand low power, and two of said plurality of open/close valves open atlow power.
 35. The turbogenerator of claim 31 wherein said valving is aplurality of three-way valves and a plurality of open/close valves. 36.The turbogenerator of claim 31 wherein said valving is a plurality offour-way valves.
 37. A turbogenerator comprising: a gas turbineincluding a compressor, a turbine, a combustor, a recuperator, a freeturbine, and a generator; said recuperator receiving compressed air fromsaid compressor to be heated in said recuperator by the expanded exhaustgases from said free turbine, said heated compressed air from saidrecuperator supplied to said combustor to be mixed with fuel andcombusted to provide combustion gases for expansion first in saidturbine which drives said compressor and then further expanded in saidfree turbine; valving disposed between said recuperator and saidcompressor to operate said turbogenerator in a positive pressure mode athigh power, and to operate said gas turbogenerator in a subatmosphericpressure mode at low power.
 38. The turbogenerator of claim 37 whereinsaid generator is driven by said turbine.
 39. The turbogenerator ofclaim 37 wherein said generator is driven by said free turbine.
 40. Theturbogenerator of claim 37 wherein said valving is a plurality ofopen/close valves.
 41. The turbogenerator of claim 40 wherein saidplurality of open/close valves is six, with three of said plurality ofopen/close valves open at high power and the other three of saidplurality of open/close valves open at low power.
 42. The turbogeneratorof claim 41 wherein said three of said plurality of open/valves open athigh power are at the recuperator exhaust, the compressor inlet, andbetween said compressor and said recuperator, and the other three ofsaid plurality of open/close valves open at low power are at therecuperator inlet, the compressor outlet and between the recuperatorexhaust and the compressor inlet.
 43. The turbogenerator of claim 37wherein said valving is a plurality of three-way valves and a pluralityof open/close valves.
 44. The turbogenerator of claim 37 wherein saidvalving is a plurality of four-way valves.
 45. A two spoolturbogenerator comprising: a first spool including a first compressor, afirst turbine, and a generator with said first turbine driving saidfirst compressor and said generator; a second spool including a secondcompressor and a second turbine with said second turbine driving saidsecond compressor; said first compressor providing compressed air tosaid second compressor; a combustor to provide combustion products tosaid second turbine which in turn provides a turbine exhaust to saidfirst turbine; a recuperator to receive compressed air from said secondcompressor to be heated in said recuperator by the expanded exhaustgases from said first turbine; said heated compressed air from saidrecuperator supplied to said combustor to be mixed with fuel andcombusted to provide combustion gases for expansion in said secondturbine and in said first turbine; and valving disposed between saidrecuperator, said first compressor, and said second compressor tooperate said turbogenerator in a positive pressure mode at high power,to operate said turbogenerator in a transatmospheric pressure mode atmid power, and to operate said turbogenerator in a subatmosphericpressure mode at low power.
 46. The turbogenerator of claim 45, and inaddition, an intercooler disposed between said recuperator and saidfirst compressor.
 47. The turbogenerator of claim 45, and in addition,an intercooler disposed between said first compressor and said secondcompressor.
 48. The turbogenerator of claim 45, and in addition, a firstintercooler disposed between said recuperator and said first compressor,and a second intercooler disposed between said first compressor and saidsecond compressor.
 49. The turbogenerator of claim 45 wherein saidvalving is a plurality of open/close valves.
 50. The turbogenerator ofclaim 49 wherein said plurality of open/close valves is nine, with twoof said plurality of open/close valves open at high power, one of saidplurality of open/close valves open at both high and mid power, one ofsaid plurality of open/close valves open at both high and low power, twoof said plurality of open/close valves open at mid power, one of saidplurality of open/close valves open at both mid and low power, and twoof said plurality of open/close valves open at low power.
 51. A two andone half spool turbogenerator comprising: a first spool including afirst compressor and a first turbine with said first turbine drivingsaid first compressor and; a second spool including a second compressorand a second turbine with said second turbine driving said secondcompressor; a free turbine and a generator with said free turbinedriving said generator; said first compressor providing compressed airto said second compressor; a combustor to provide combustion products tosaid second turbine which in turn provides turbine exhaust to said firstturbine which in turn provides turbine exhaust to said free turbine; arecuperator to receive compressed air from said second compressor to beheated in said recuperator by the expanded exhaust gases from said freeturbine; said heated compressed air from said recuperator supplied tosaid combustor to be mixed with fuel and combusted to provide combustiongases for expansion in said second turbine, said first turbine and saidfree turbine; and valving disposed between said recuperator, said firstcompressor, and said second compressor to operate said turbogenerator ina positive pressure mode at high power, to operate said turbogeneratorin a transatmospheric pressure mode at mid power, and to operate saidturbogenerator in a subatmospheric pressure mode at low power.
 52. Theturbogenerator of claim 51, and in addition, an intercooler disposedbetween said recuperator and said first compressor.
 53. Theturbogenerator of claim 51, and in addition, an intercooler disposedbetween said first compressor and said second compressor.
 54. Theturbogenerator of claim 51, and in addition, a first intercoolerdisposed between said recuperator and said first compressor, and asecond intercooler disposed between said first compressor and saidsecond compressor.
 55. The turbogenerator of claim 51 wherein saidvalving is a plurality of open/close valves.
 56. The turbogenerator ofclaim 55 wherein said plurality of open/close valves is nine, with twoof said plurality of open/close valves open at high power, one of saidplurality of open/close valves open at both high and mid power, one ofsaid plurality of open/close valves open at both high and low power, twoof said plurality of open/close valves open at mid power, one of saidplurality of open/close valves open at both mid and low power, and twoof said plurality of open/close valves open at low power.
 57. Theturbogenerator of claim 51 wherein said valving includes a plurality ofthree-way valves.
 58. The turbogenerator of claim 51 wherein saidvalving includes a plurality of three-way valves and a plurality ofopen/close valves.
 59. The turbogenerator of claim 58 wherein saidplurality of three-way valves is four and said plurality of open/closevalves is two.
 60. The turbogenerator of claim 59 wherein said fourthree-way valves are at the recuperator inlet, the recuperator exhaust,the first compressor inlet and said first compressor exhaust, and saidtwo open/close valves are between the recuperator inlet three-way valveand said second compressor, and between said second compressor and saidintercooler between said first compressor and said second compressor.61. The turbogenerator of claim 54 wherein said valving is a pluralityof four-way valves.
 62. The turbogenerator of claim 61 wherein saidplurality of four-way valves is four.
 63. The turbogenerator of claim 62wherein said four four-way valves are at the recuperator inlet, therecuperator outlet, the first compressor inlet, and said firstcompressor outlet.
 64. The turbogenerator of claim 63 wherein inpositive pressure mode, said recuperator inlet four-way valve connectssaid second compressor outlet to said recuperator high pressure inlet,said recuperator outlet four-way valve connects said recuperator lowpressure outlet to ambient, said first compressor inlet four-way valveconnects said first compressor inlet to ambient, and, said firstcompressor outlet four-way valve connects said first compressor outletto said second compressor inlet through said second intercooler.
 65. Theturbogenerator of claim 63 wherein in transatmospheric pressure mode,said recuperator inlet four-way valve connects said second compressoroutlet to said recuperator high pressure inlet, said recuperator outletfour-way valve connects said recuperator low pressure outlet to saidfirst compressor inlet through said first intercooler, said firstcompressor inlet four-way valve connects said recuperator low pressureoutlet to said first compressor inlet through said first intercooler,and, said first compressor outlet four-way valve connects said firstcompressor outlet to ambient and said second compressor inlet to ambientthrough said second intercooler.
 66. The turbogenerator of claim 63wherein in subatmospheric pressure mode, said recuperator inlet four-wayvalve connects said recuperator high pressure inlet and said secondcompressor outlet to ambient, said recuperator outlet four-way valveconnects said recuperator low pressure outlet to said first compressorinlet through said first intercooler, said first compressor inletfour-way valve connects said recuperator low pressure outlet to saidfirst compressor inlet through said first intercooler, and, said firstcompressor outlet four-way valve connects said first compressor outletto said second compressor inlet through said second intercooler.
 67. Atwo and one half spool turbogenerator comprising: a first spoolincluding a first compressor and a first turbine with said first turbinedriving said first compressor and; a second spool including a secondcompressor and a second turbine with said second turbine driving saidsecond compressor; a free turbine and a generator with said free turbinedriving said generator; said first compressor providing compressed airto said second compressor; a combustor to provide combustion products tosaid second turbine which in turn provides turbine exhaust to said freeturbine which in turn provides turbine exhaust to said first turbine; arecuperator to receive compressed air from said second compressor to beheated in said recuperator by the expanded exhaust gases from said firstturbine; said heated compressed air from said recuperator supplied tosaid combustor to be mixed with fuel and combusted to provide combustiongases for expansion in said second turbine, said free turbine, and saidfirst turbine; and valving disposed between said recuperator, said firstcompressor, and said second compressor to operate said turbogenerator ina positive pressure mode at high power, to operate said turbogeneratorin a transatmospheric pressure mode at mid power, and to operate saidturbogenerator in a subatmospheric pressure mode at low power.
 68. Theturbogenerator of claim 67, and in addition, a first intercoolerdisposed between said recuperator and said first compressor, and asecond intercooler disposed between said first compressor and saidsecond compressor.
 69. The turbogenerator of claim 68 wherein saidvalving is nine open/close valves with two of said plurality ofopen/close valves open at high power, one of said plurality ofopen/close valves open at both high and mid power, one of said pluralityof open/close valves open at both high and low power, two of saidplurality of open/close valves open at mid power, one of said pluralityof open/close valves open at both mid and low power, and two of saidplurality of open/close valves open at low power.
 70. The turbogeneratorof claim 68 wherein said valving includes four three-way valves and twoopen/close valves.
 71. The turbogenerator of claim 70 wherein said fourthree-way valves are at the recuperator inlet, the recuperator exhaust,the first compressor inlet and said first compressor exhaust, and saidtwo open/close valves are between the recuperator inlet three-way valveand said second compressor, and between said second compressor and saidintercooler between said first compressor and said second compressor.72. The turbogenerator of claim 68 wherein said valving is four four-wayvalves.
 73. The turbogenerator of claim 72 wherein said four four-wayvalves are at the recuperator inlet, the recuperator outlet, the firstcompressor inlet, and said first compressor outlet.
 74. Theturbogenerator of claim 73 wherein in positive pressure mode, saidrecuperator inlet four-way valve connects said second compressor outletto said recuperator high pressure inlet, said recuperator outletfour-way valve connects said recuperator low pressure outlet to ambient,said first compressor inlet four-way valve connects said firstcompressor inlet to ambient, and, said first compressor outlet four-wayvalve connects said first compressor outlet to said second compressorinlet through said second intercooler.
 75. The turbogenerator of claim73 wherein in transatmospheric pressure mode, said recuperator inletfour-way valve connects said second compressor outlet to saidrecuperator high pressure inlet, said recuperator outlet four-way valveconnects said recuperator low pressure outlet to said first compressorinlet through said first intercooler, said first compressor inletfour-way valve connects said recuperator low pressure outlet to saidfirst compressor inlet through said first intercooler, and, said firstcompressor outlet four-way valve connects said first compressor outletto ambient and said second compressor inlet to ambient through saidsecond intercooler.
 76. The turbogenerator of claim 73 wherein insubatmospheric pressure mode, said recuperator inlet four-way valveconnects said recuperator high pressure inlet and said second compressoroutlet to ambient, said recuperator outlet four-way valve connects saidrecuperator low pressure outlet to said first compressor inlet throughsaid first intercooler, said first compressor inlet four-way valveconnects said recuperator low pressure outlet to said first compressorinlet through said first intercooler, and, said first compressor outletfour-way valve connects said first compressor outlet to said secondcompressor inlet through said second intercooler.
 77. A two spoolturbogenerator comprising: a first spool including a first compressor, afirst turbine, and a first generator with said first turbine drivingsaid first compressor and said generator; a second spool including asecond compressor, a second turbine, and a second generator with saidsecond turbine driving said second compressor and said second generator;said first compressor providing compressed air to said secondcompressor; a combustor to provide combustion products to said secondturbine which in turn provides a turbine exhaust to said first turbine;a recuperator to receive compressed air from said second compressor tobe heated in said recuperator by the expanded exhaust gases from saidfirst turbine; said heated compressed air from said recuperator suppliedto said combustor to be mixed with fuel and combusted to providecombustion gases for expansion in said second turbine and said firstturbine; and valving disposed between said recuperator, said firstcompressor, and said second compressor to operate said turbogenerator ina positive pressure mode at high power, to operate said turbogeneratorin a transatmospheric pressure mode at mid power, and to operate saidturbogenerator in a subatmospheric pressure mode at low power.
 78. Theturbogenerator of claim 77, and in addition, an intercooler disposedbetween said first compressor and said second compressor.
 79. Theturbogenerator of claim 77, and in addition, a second combustor disposedbetween said second turbine and said first turbine.